[11] _E_0 is the energy taken up, as judged from a co-ordinate system
moving with the body.
In consideration of the expression given above for the kinetic energy
of the body, the required energy of the body comes out to be
image025
Thus the body has the same energy as a body of mass
image026
moving with the velocity _v_. Hence we can say: If a body takes up an
amount of energy _E_0, then its inertial mass increases by an amount
image027
the inertial mass of a body is not a constant but varies according to
the change in the energy of the body. The inertial mass of a system of
bodies can even be regarded as a measure of its energy. The law of the
conservation of the mass of a system becomes identical with the law of
the conservation of energy, and is only valid provided that the system
neither takes up nor sends out energy. Writing the expression for the
energy in the form
image028
we see that the term _mc_2, which has hitherto attracted our attention,
is nothing else than the energy possessed by the body[12] before it
absorbed the energy _E_0.
35